The background information and description of the controllable partial rebreathing anesthesia circuit as contained in the incorporated reference is useful in obtaining a full understanding of the present invention; however, such background information will not be repeated herein.
Prior to the present invention, the common method of determining arterial carbon dioxide tension was to draw a specimen of the blood from the artery and take the specimen to a laboratory. In the laboratory, the specimen of blood would be placed in a blood gas analyzer. As part of the blood gas analyzer, the specimen of blood would flow over the end or tip of the Severinghaus electrode. A semipermeable membrane at the tip of the Severinghaus electrode would allow the gases contained in the specimen to penetrate therethrough. Inside of the Severinghaus electrode, the carbon dioxide reacts with a bicarbonate solution (NaHCO.sub.3) to give off a hydrogen ion (H+). The hydrogen ion diffuses into a glass pH electrode located in the center of the cylindrically shaped Severinghaus electrode. The hydrogen ion creates a voltage potential inside of the pH electrode, which voltage potential has an extremely high impedance, normally in the range of 10.sup.12 -10.sup.15 ohms. Proper shielding for the output of the Severinghaus electrode becomes very critical because of the high impedance. The output is proportional to the carbon dioxide content of the blood.
After the measurement of the blood gases as just described, the results have to be transmitted back to the requesting physician. The typical time delay between the request for blood gas analysis, and getting the analysis back, is approximately 15-20 minutes. One problem that may occur as a result of drawing arterial blood gas specimens is arterial occlusion. Arterial occlusion may cause:
1. Ischemia or loss of fingers and/or thumb; or PA0 2. Loss of function of the small muscles of the hands.
Another problem that may occur is infection at the site of the puncture. Still another major problem associated with arterial lines is air embolism.
An additional advantage of the present invention over prior methods of determining blood gases by taking specimens of blood from an individual's artery is the cost. In cases where numerous blood gas samples are required over a period of time, each of the blood gas samples has the normal high cost associated with laboratory analysis.
In small children, there are numerous problems in the drawing of blood gases, including the inaccessibility of the small blood vessels of the child. In cases where the patient is not under anesthesia at the time, considerable pain is associated with the drawing of blood gases.
Other methods of analyzing carbon dioxide in patients during anesthesia include the use of mass spectrophotometry or capnographs. The disadvantages of mass spectrophotometry are its size and expense. The mass spectrophotometry equipment is not designed for use in the operating room where space is at a premium. In the case of infrared capnographs, the cost is usually prohibitive for use by each and every patient in the operating room. The instrument is likewise large, cumbersome and bulky. It is also not designed to be used in the operating room. In addition, an infrared capnograph analysis of CO.sub.2 is influenced by nitrous oxide, which is frequently used in combination with other anesthetics or alone by patients receiving anesthesia, thereby causing error in results obtained.